My guess is owning the user time, e.g if you read article via AMP you don't even visit the original website anymore. Same with providing answers directly in the search results.

The long term benefits are that they stem the bleeding of everything moving inside mobile apps. They need content on the web to remain useful and developers have shown they're unable to make good mobile experiences themselves.

I think this particular move is more tactical than strategic. I think it provides a better experience for users and keeps Pinterest, one of the main AMP users, happy with Google.

Get 'em hooked on something 'free', then monetize it. Rinse and repeat. Just like they are building a browser that only blocks the 'annoying ads', now they are designing their email to let advertising through. So much for 'don't be evil'.

Total eco system control, offering advertisers and companies more ways to market and sell, being able to push their own ad platform and ads.